ULC-BH钢磷、碳晶界偏聚及对{111}面织构与烘烤硬化性能影响的研究

The ultra-low-carbon bake-hardening (ULC-BH) steel has demonstrated superior overall performance and is currently used widely in the modern automotive industry for exposed panels. Grain boundary segregation of solute elements will result in unstable performance of the steel in the annealing process. Hence, the effect of phosphorus and carbon grain boundary segregation on the property of ULC-BH steel has become an urgent problem that needs to be addressed. In this project, the phosphorus and carbon grain boundary segregation models for ULC-BH steels with different phosphorus contents are constructed, focusing on two aspects: one is investing in the mutual relationship of grain boundary segregation between phosphorus atoms and carbon atoms; the other is revealing the mechanism of phosphorus and carbon grain boundary segregation in ULC-BH steel. Advanced experimental technologies, such as EBSD ,3DAPand TEM, will be used to investigate the microstructure texture evolution and the distribution pattern of solution elements of the sample after recrystallization annealing. Moreover, the microstructure and dislocation structure will also be observed using a microscope. The main objective is to explore the effect of phosphorus and carbon grain boundary segregation on {111} texture, deep drawing property, and bake-hardening property of ULC-BH steel, as well as research on the precipitation behavior of steel in the annealing process. The correlation is established among annealing parameters, phosphorus and carbon grain boundary segregation, and overall performance of ULC-BH steel. This project aims to provide an experimental and theoretical basis for the development and application of ULC-BH steel.

超低碳烘烤硬化（ULC-BH）钢因具有优良的综合性能，而被广泛应用于汽车面板，然而在退火过程中磷、碳易偏聚于晶界导致钢板性能不稳，因此需要对磷、碳晶界偏聚机理进行深入研究。本项目以不同磷含量的ULC-BH钢为研究对象，构建磷晶界偏聚、碳晶界偏聚及磷/碳晶界共偏聚模型，探讨磷/碳晶界偏聚的相互作用机制，揭示ULC-BH钢中磷、碳晶界偏聚机理；采用EBSD、3DAP及TEM等测试技术分析退火前后试样微观织构的演变规律，磷与碳元素在基体中的分布特征及微观组织和位错结构的变化趋势，结合晶界偏聚模型阐明磷的晶界偏聚对ULC-BH钢{111}面织构与深冲性能的影响规律和机理，明确碳的晶界偏聚与材料烘烤硬化性能的关系及微观作用机制，解析该材料在退火过程中析出行为，建立退火参数、磷/碳的晶界偏聚与ULC-BH钢综合性能的相关性。项目研究成果可为高性能ULC-BH钢的研发与应用提供实验依据和理论指导。

添加磷高强ULC-BH钢退火过程中磷与碳原子晶界偏聚行为尚不清楚，磷添加对ULC-BH钢{111}面织构及烘烤硬化性能的影响机制仍存在争议。本项目利用3DAP、EBSD、内耗及TEM等测试技术并结合相关理论，研究添加不同磷含量的ULC-BH钢在退火过程中P晶界偏聚、C晶界偏聚以及二者共偏聚行为，揭示磷元素的添加对ULC-BH钢{111}面织构影响规律及微观作用机制，阐明了磷与碳原子晶界偏聚对ULC-BH钢烘烤硬化行为的影响机理。探究添加钛、钒及磷元素ULC-BH钢的析出行为及其对材料性能的影响。其研究成果不仅为ULC-BH钢成分设计提供新的思路，也为提高ULC-BH钢的成形性能与烘烤硬化性能提供重要参考。通过本项目的研究获得了如下有意义的研究成果: （1）超低碳钢与低碳钢中磷原子与碳原子晶界偏聚关系正好相反，对于ULC-BH钢，磷的晶界偏聚抑制碳晶界偏聚，因而，ULC-BH钢中基体固溶碳浓度大于未含磷的ULC-BH钢，这一发现可为ULC-BH钢成分设计提供理论与试验支撑。（2）磷元素的添加可在较大程度上提高ULC-BH钢烘烤硬化性能，随着预应变的增加，含磷ULC-BH钢烘烤硬化性能不断提高，而普通ULC-BH钢烘烤硬化值先增大后降低，预应变对烘烤硬化性能影响主要取决于基体中固溶碳浓度和位错密度值。（3）磷元素的添加在一定程度上抑制ULC-BH钢再结晶过程并细化钢的晶粒尺寸，在750℃退火时，含磷ULC-BH钢{111}面织构大于未含磷ULC-BH钢，而在810℃退火时，结果正好相反，P的添加抑制{111}面织构的发展。研究发现对于ULC-BH钢，存在一“补偿温度”，高于或低于补偿温度下退火时磷晶界偏聚对{111}面织构的影响规律有所不同，因此在研究P添加对退火织构影响时必须考虑退火温度这一重要因素，此研究结果为高强度高成形性能含磷ULC-BH钢的开发提供一个可行途径。